The present invention relates to processes for applying catalyst or catalyst support coatings onto ceramic supports. More particularly, the invention relates to methods for coating ceramic substrates with catalyst coatings wherein a pre-coating or passivation step is used to improve the properties of the catalyzed substrates, by reducing catalyst and/or support coating diffusion into the fine pores, microchannels (necks interconnecting individual pores), and microcrack structure of the substrates.
To address tightening diesel engine emissions regulations being adopted in the United States and Europe, recent attention has focused on basic improvements in the design and performance of ceramic wall-flow honeycomb filters for treating diesel exhaust gases. Among other improvements, design changes allowing for the use of catalyst coatings to control hydrocarbon and/or nitrogen oxide emissions are being implemented. The goal is to develop an improved high-temperature-resistant, high-thermal-shock-resistant, low cost honeycomb soot filter compatible with advanced emissions control catalyst technologies that can replace current high-cost and/or uncatalyzed particulate filters.
Among the filter designs being developed for this application are refractory ceramic oxide filters offering improved resistance to high exhaust temperatures encountered during decarbonizing filter regeneration cycles, as well as to the thermal shock conditions arising during rapid filter heat-up and cool-down in the course of startup and regeneration. Examples of advanced cordierite and aluminum titanate compositions and honeycomb filter designs being developed for these applications are disclosed in U.S. Pat. No. 6,541,407 and in co-pending, commonly assigned U.S. Patent Applications Ser. Nos. 60/400,248 filed Jul. 31, 2002, Ser. No. 10/209,684 filed Jul. 31, 2002, and Ser. No. 10/098,711 filed Mar. 14, 2002. Among other materials that are candidates for refractory, catalyst-compatible ceramic particulate filters are the refractory alkali zirconium phosphates as well as low-expansion alkali aluminosilicates such as beta-eucryptite and pollucite. Many of these same compositions, and other microcracked ceramic materials such as the calcium aluminates, are being considered for use as flow-through catalyst supports for the control of nitrogen oxide (NOx) emissions from automotive and diesel engines
These ceramic materials meet or exceed most specifications for high melting point, high thermal capacity, and low thermal expansion required for diesel exhaust filter applications. However, one difficulty encountered with porous ceramics intended to function as particulate filters is the tendency to decrease in gas permeability and increase in thermal expansion as catalysts and catalyst support washcoatings are applied to the filter walls. For good thermal shock resistance, the increases in CTE resulting from the application of washcoats and catalysts should not exceed 10×10−7/° C. averaged over the range from 25–1000° C., and CTE values for the washcoated filters should not exceed 20×10−7/° C. over that temperature range. Further, gas permeabilities through the catalyzed filter should be sufficient to maintain pressure drops below 8 kPa at exhaust gas space velocities up to 150,000 hr−1 after filter regeneration to remove trapped particulates.
Present understanding is that, during the washcoating or catalyzing process, both wall porosity of the filter and the structural micro-cracks (crack widths of 0.1–3 microns) that are present in most of these ceramic materials are frequently filled with the washcoating material. The problem is most severe in the case of highly microcracked ceramics such as the aluminum titanates, particularly when the washcoating formulations contain materials of very fine particle size (e.g., particle diameters in the 0.02–0.1 μm range).
Microcracking is a significant contributor to the low CTEs exhibited by many of these materials, with crack closure during heating considerably moderating the dimensional increases that would otherwise occur. Thus the filling of these microcracks with washcoating constituents can result in some cases in much higher expansion coefficients, e.g., in the range of 40–50×10−7/° C., in the washcoated structures. At these CTE levels the risk of structural damage to the filters under the normal conditions of exhaust filter use is unacceptable.
One approach to the problem of washcoat microcrack filling that has been employed during the catalyzation of conventional flow-through catalyst substrates for gasoline engine emissions control has been the use of so-called passivating coatings. These are pre-coatings applied to the walls of the ceramic substrates prior to washcoating that can block the washcoating materials from intruding into the microcrack structure of the ceramic. U.S. Pat. No. 4,532,228 provides some examples of coating materials that can be carbonized or otherwise solidified to provide a washcoat barrier.
Recent advances in materials and processes for the pre-washcoat passivation of microcracked ceramic wall flow filters and flow-through catalyst supports include those described in co-pending, commonly assigned U.S. patent application Ser. No. 10/641,638 of S. Ogunwumi et al., filed Aug. 14, 2003, expressly incorporated herein by reference in its entirety. That application discloses pre-coating such ceramics with polymer barrier or passivation layers to prevent washcoat nanoparticle intrusion into the microcracked and/or microporous surfaces of the ceramics. The barrier coatings employed are formed of hydrocarbon polymers that are soluble or dispersible in polar media, capable of forming neutral or hydrophilic surfaces on porous ceramic supports, and completely vaporizable at moderate washcoat stabilization or catalyst activation temperatures.
The polymer barrier coatings disclosed in that application moderate CTE increases and limit the reductions in exhaust gas permeability necessarily arising from the application of washcoating layers to microporous ceramic filters, although in some cases surface interactions between the hydrophobic polymer coatings and the hydrophilic washcoat are observed. Thus while backpressure increases from washcoat layering and pore blockage are difficult to avoid entirely, the described coatings offer improvements in thermal expansion and gas permeability characteristics that enable a wide variety of ceramic filter materials to meet existing commercial requirements.
Nevertheless, although progress in development of catalytic filters has been substantial, thermal expansion and pressure drop and/or porosity remain key performance characteristics, not only of catalytic filters but also of conventional flow-through ceramic catalyst supports as well. Thus improved materials and methods for simultaneously preserving the low thermal expansion coefficients and high gas permeabilities of advanced ceramic support or filter materials, even at high catalyst washcoat loadings, remain important objectives of current development programs.